This project tests the functions of immunoglobulin G (IgG) antibodies in suppressing the development of food allergy and in restoring tolerance in the setting of established allergy. Our preliminary data that IgG against peanut prevents sensitization to peanut, and we hypothesize that this may be due to silencing the activation of mast cells, immune cells which we have previously shown to amplify the development of the allergic response. We will investigate whether IgG requires the inhibitory receptor, Fc?R2b, in order to suppress the allergic response. We will also investigate the effects of IgG on dendritic cells, which are the initiators of immune responses and carry both inhibitory Fc?R2b and activating Fc?R3 receptors for IgG. The contributions of the positive and negative signals from these receptors will be corroborated by examining the importance of key signaling molecules (Syk, Shp1) downstream of the receptors inside the cells. These objectives will be tested using a new robust mouse model of peanut allergy in which the mice exhibit sensitization and anaphylaxis similar to that seen in human patients. The functions of specific immune cells and molecules will be tested using genetically engineered mice and pharmaceutical approaches. The specific aims are as follows: AIM 1: To determine whether Fc?R2b cancels the adjuvant effect of mast cells. The hypothesis being tested is that IgG, binding to Fc?R2b on mast cells, produces inhibitory signals to restore the Treg:Th2 balance. AIM 2: To elucidate the mechanisms of IgG-enhanced tolerance induction to ingested allergens. The hypothesis being tested is that IgG ligates Fc?R2b on intestinal dendritic cells, acting via Shp-1 to promote oral tolerance, while limiting pro-allergenic cytokines induced by Fc?R3 and Syk. AIM 3: To test the cellular and molecular mechanisms of peanut-specific IgG as an adjunct to conventional OIT. The hypothesis being tested is that IgG, acting via Fc?R2b, will enhance Treg responses while suppressing Th2 responses. This project is expected to generate critical new information regarding the basic mechanisms of immune sensitization to food allergens occurring in the gastrointestinal tract. These data will lay the groundwork for the development of new, mechanism-based therapies for food allergy, a disorder that is rapidly increasing in developed nations and has no approved treatments. Importantly, this proposal will expand our knowledge of the actions of IgG in food allergy, and may provide a rational basis for its use as a therapy.